1. Field of the Invention
The present invention relates to an image coding apparatus and image coding method, image decoding apparatus and image decoding method, recording medium, image transmitting method and image processing apparatus, and more particularly relates to the image coding apparatus and image coding method, image decoding apparatus and image decoding method, recording medium, image transmitting method and image processing apparatus capable of thinning-out (subsampling) and compression encoding an image in such a manner that a decoded image is almost identical to a source image.
2. Description of the Related Art
Conventionally, various methods have been proposed as methods for compressing images. One such method employs the original image (i.e., the image to be encoded) as an image of a "first hierarchy" (uppermost hierarchy). An image of a second hierarchy and an image of a third hierarchy are formed by reducing the number of pixels (i.e., resolution is successively lowered).
In accordance with a conventional hierarchical coding system, images of plural hierarchies are transmitted from a transmitting apparatus to a receiving apparatus. At the receiving apparatus, images of the respective hierarchies may be displayed (e.g., on a monitor) in response to each of these images of the plural hierarchies.
Moreover, in accordance with a conventional hierarchical decoding system, an error correction process operation is carried out for data about the image in the lowermost hierarchy (namely, the image with the lowest resolution) but not for the images of the other higher hierarchies. As a result, when an error happens to occur, no error correction can be made as to the images of the hierarchies other than the lowermost hierarchy. As a result, under any error condition, only the data of the image of the lowermost hierarchy can be acquired and corrected. Images of the hierarchies higher than the lowermost hierarchy do not have data for error recovery and, thus, may be obtained only by way of, for example, an interpolation process operation from the data about the image of the lowermost hierarchy on the receiver end. Therefore, the robustness characteristic with respect to the error can be improved in accordance with the hierarchical coding system.
FIG. 28 illustrates one example of a conventional image coding apparatus for performing the above-described hierarchical coding operation. Image data to be coded is supplied as data of a first hierarchy (uppermost hierarchy) to a thinning-out (sub-sampling) unit 11.sub.1 and an arithmetic unit 12.sub.1 as data for a first hierarchical layer (uppermost hierarchical layer).
At the thinning-out unit 11.sub.1, image data for a second hierarchical layer (one order down) is formed by thinning out the number of pixels of the image data for the first hierarchical layer and supplying the resulting pixels to a thinning-out unit 11.sub.2, arithmetic unit 12.sub.2 and an interpolator 501.sub.1. At the interpolator 501.sub.1, image data (hereinafter referred to as "first hierarchical layer interpolation data") is formed of the same number of pixels as the image data for the first hierarchical layer (one order up) and supplied to the arithmetic unit 12.sub.1. The first hierarchical layer interpolation data is then subtracted from the first hierarchical layer image data at the arithmetic unit 12.sub.1 and the resulting difference value is output to a signal processing unit 502 as coded data for the first hierarchical layer.
Similar processing is carried out at a thinning-out unit 11.sub.2, an arithmetic unit 12.sub.2 and an interpolator 501.sub.2 ; a thinning-out unit 11.sub.3, an arithmetic unit 12.sub.3 and an interpolator 501.sub.3 ; and a thinning-out unit 11.sub.4, an arithmetic unit 12.sub.4 and an interpolator 501.sub.4. As a result, coded data is generated for the second to fourth hierarchical layers, respectively, and output to the signal processing unit 502 from the arithmetic units 12.sub.2 to 12.sub.4, respectively.
Image data for the fifth hierarchical layer (lowermost hierarchical layer) formed at the thinning-out unit 11.sub.4 is output to the signal processing unit 50.sub.2, without modification, as coded data for the fifth hierarchical layer.
At the signal processing unit 502, error correction and other necessary signal processing is performed on the coded data for the first to fifth hierarchical layers. After this, the resulting processed data is multiplexed and outputted as the final coded data. At the signal processing unit 502, stronger error correction is performed on coded data for the fifth hierarchical layer than for other hierarchical layers.
FIG. 29 illustrates an example embodiment of an image decoding device for hierarchically decoding coded data output from the image coding device of FIG. 28.
At a signal processing unit 602, coded data is divided into coded data for the first to fifth hierarchical layers. Also, error correction processing and other necessary processing is performed, and the processed data is output. Coded data for the first to fourth hierarchical layers is supplied to arithmetic units 73.sub.1 to 73.sub.4. The coded data for the fifth hierarchical layer is provided as the decoded image for the fifth hierarchical layer, and supplied to an interpolator 601.sub.4.
The interpolator 601.sub.4 corresponds to the interpolator 501.sub.4 of FIG. 28. Therefore, image data of the same number of pixels as the image data for the fourth hierarchical layer (of one order up) i.e. interpolation data for the fourth hierarchical layer can be generated by performing interpolation processing on the coded data for the fifth hierarchical layer, and this data is supplied to the arithmetic unit 73.sub.4. The coded data for the fourth hierarchical layer (including an error value for the image data for the fourth hierarchical layer and the interpolation data for the fourth hierarchical layer) and interpolation data for the fourth hierarchical layer from the interpolator 601.sub.4 are added at the arithmetic unit 73.sub.4. The results of this addition are then output as the decoded image for the fourth hierarchical layer and also supplied to the interpolator 601.sup.3.
A similar process as is carried out at the arithmetic unit 73.sub.4 and the interpolator 601.sub.4 is also carried out at the arithmetic unit 73.sub.3 and the interpolator 601.sub.3 ; the arithmetic unit 73.sub.2 and the interpolator 6012; and the arithmetic unit 73.sub.1 and the interpolator 601.sub.1 so that decoded images are also generated for the third to first hierarchical layers and outputted from the arithmetic units 73.sub.3 to 73.sub.1, respectively.
When coded data for the first to fourth hierarchical layers cannot be obtained at the image decoding device for some reason, interpolation can be carried out just using the coded data for the fifth hierarchical layer if the coded data for the fifth hierarchical layer can be obtained and a decoded image for each of the hierarchical layers can be obtained.
However, the picture quality of decoded images obtained from just the coded data for the lowermost hierarchical layer is extremely inferior.